In recent years, a water-saving type washing machine, i.e. a washing machine of which amount of water used for washing is small, has become widespread, from an increasing awareness of environmental issues, or the like. A marked example thereof is a washing machine generally so-called “high efficiency-type washer, drum-type front-load washer in Japanese,” which has been broadly accepted worldwide. However, this washing system in which a smaller amount of water is used against a washing item has a disadvantage that a washing item is gradually looking gray by performing washing. The reduction in a washing water is preferable from the viewpoint of reduction of loads on environmental issues by conservation of energy, economic advantages, and the like. However, one of the main causations of the “gray-looking” of a washing item is the small amount of the used water as mentioned below; therefore, improvements by the washing machine itself have been a difficult disadvantage. Therefore, at present with the progress in the widespread use of a water-saving type washing machine, a detergent composition not only exhibiting high detergency but also being capable of controlling the gray-looking of a washing item, even in a washing system with a smaller amount of water used as described above has been earnestly desired.
A typical phenomenon of gradual gray-looking of a washing item by every wash includes a phenomenon wherein colored components such as soils that are detached from a washing item during the washing process are redeposited to the washing item in a washing water again. When the amount of the washing water becomes smaller, the soil concentration in the washing water increases, so that such soils are more likely to be redeposited. Even in a case where a concentration of a detergent or a dispersant, such as a surfactant, as well as soils, increases in a washing liquid by reducing the amount of water, a negative effect caused by an increase in the soil concentration in water is greater than a positive effect of preventing redeposition by these components; therefore, a phenomenon of causing acceleration of the soil redeposition to the washing item can also be confirmed.
In addition, among the soils in a washing liquid, representative examples of colored components are fine hydrophilic particles such as those contained in sludge, and fine hydrophobic particles such as soot. In order to prevent gray-looking, a detergent composition having high deposition preventing ability against soil particles having different physical properties from those mentioned above is required.
Conventionally, as a technique of preventing redeposition, as described in Patent Publication 1, one having an improved dispersibility of soils in a washing liquid with a dispersant such as a polymer has been known.
In addition, a technique based on a mechanism different from that of above is such that the physical properties of the surfaces of the fibers are modified by adsorbing a specified chemical species to fibers, and whereby the redeposition of the soils are prevented. For example, Patent Publication 2 describes that the redeposition preventing ability of soils is improved by depositing a smectite-type clay mineral to the fibers. However, in the case of Patent Publication 2, in a mechanism for preventing redeposition such as one exhibiting the effect by deposition of the chemical species, a mechanism of stably dispersing soil particles in a washing liquid is not suggested, so that any effects in a case where deposition to fibers of the chemical species is undesired are hardly exhibited. Particularly, a surfactant, which is a technique concerning dispersion systems, especially a nonionic surfactant, is not suggested, so that redeposition preventing ability of fine hydrophobic particles such as soot, that is considered to be an important factor for preventing gray-looking, is not sufficient. In addition, in this mechanism, the repetition of washes is a necessary condition for exhibiting the effect, so that it is not sufficient to prevent gray-looking in a high-soil concentration liquid mixture such as one in which gray-looking is recognized even in a single cycle of wash.
In addition, a technique utilizing a smectite-type clay mineral, as described in Patent Publication 3, includes an example using a clay mineral in order to improve the physical properties of granules of a detergent composition containing a nonionic surfactant. However, in the case of Patent Publication 3, since there is no technical idea of preventing gray-looking of a washing item, the content of an anionic surfactant is not sufficient, so that the redeposition preventing ability of fine hydrophilic particles such as sludge particles, one of the causations of the gray-looking, is not sufficient.
A technique of soil preventing treatment of laundry in the step of washing the laundry with a washing machine by applying an amphoteric polymer having a cationic group and an anionic group to a laundry detergent composition has been known (Patent Publication 4). However, while a detergent effect for sebum soils during washing is high, redeposition preventing ability of fine hydrophilic particles as those contained in sludge and fine hydrophobic particles such as soot by the soils in a washing liquid is not sufficient.
Therefore, in any of these techniques, it is not sufficient to prevent the redeposition of the soils in a washing water with a drastically increased soil concentration by making the amount of the washing water small.    Patent Publication 1: JP-A-Showa-62-253694    Patent Publication 2: JP-A-Showa-56-167798    Patent Publication 3: JP-B-3043976    Patent Publication 4: JP-B-3405941